In twin spool gas turbine engines, working medium gases are compressed within a low pressure compression section and subsequently a high pressure compression section and used as an oxidizing agent in the production of a high temperature effluent. The high temperature effluent is subsequently expanded through a high pressure turbine section and subsequently through a low pressure turbine section. The high pressure turbine drives the high pressure compressor by way of a high pressure shaft and the low pressure compressor is driven by the low pressure turbine by way of a low pressure shaft disposed within the high pressure shaft. Within the turbine section rotor stages attached to the shaft are comprised of a hub, a disk and blades disposed about the peripheries of the disk. The flowpath shape is defined and maintained by a circumferential air seal between the two rotor stages. Blades extend outwardly across the flowpath for working medium gases to extract energy from the gases flowing thereacross. The energy is transmitted to the shaft by way of the disk and hub. High pressure turbines usually comprise two rotor stages with approximately equal amounts of work extracted from each rotor stage. Modern turbofan engines can generate over 60,000 pounds of thrust. The torque transmitted by each rotor stage of the high pressure turbine to the high pressure shaft in a large turbofan engine is approximately 500,000 inch pounds.
A major design goal of complicated turbofan engines is ease of assembly and disassembly while still maintaining structural integrity and limiting the weight of the engine. Limiting the size and weight of the disk portion of the turbine rotor stage while maintaining the structural integrity of the turbine rotor assembly is extremely beneficial. Eliminating holes and flanges for connecting the two turbine rotor stages together is also beneficial for preserving material strength in the face of high centrifugal loads and vibrations.
It is known in the field to attach the two rotor stages of the high pressure turbine together using either bolts or a more permanent means such as welding. It is further known to bolt or weld rotor stages to the shaft. These methods of attaching the two rotor stages to each other results in a gas turbine engine that is more complicated and more difficult to assemble and disassemble than is desired. Furthermore, the use of bolt holes in a disk and the flanges required to attach adjacent rotor stages together requires beefed up disks and heavier rotor stages. Bolt holes reduce the stress capability and structural integrity of the disks. Flanges increase the weight of the rotor stage and contribute to vibration problems that must be designed around. Prior art such as U.S. Pat. No. 3,997,962 to Kleitz et al. entitled "Method and Tool for Removing Turbine from Gas Turbine Twin Spool Engine" teaches the use of a spline to attach the two rotor stages to a single shaft. U.S. Pat. No. 4,004,860 to Gee entitled "Turbine Blade with Configured Stalk" shows the hub of the first rotor stage splined to the shaft, and the hub of the second rotor stage splined to the hub of the first rotor stage so that the shaft, the first rotor stage hub and the second rotor stage hub are all concentric. We have discovered that this type of design has difficulty maintaining concentricity between the hubs and the shaft. This means of attachment causes excessive wear of the splines thereby diminishing structural integrity of the hub to hub and the shaft to hub connections. It is also desired to be able to hold the turbine rotor assembly together so that it can be easily and safely transported for later installation in an engine.